1. Field of the Invention
The present invention relates to an anode comprising an anode current collector and an anode active material layer, and a battery using the anode.
2. Description of the Related Art
In recent years, as mobile devices have higher performance and more functions, secondary batteries with higher capacity used as power sources of the mobile devices have been desired. As a secondary battery which meets the requirement, there is a lithium secondary battery. However, in a now-typical lithium secondary battery using lithium cobalt oxide as a cathode and graphite as an anode, its battery capacity is in a saturation state, so it is extremely difficult to significantly increase the capacity. Therefore, it has been considered since a long time ago that lithium (Li) metal is used for an anode, but in order to put the anode to practical use, it is required to improve precipitation/dissolution efficiency of lithium and control dendrite deposition.
On the other hand, recently, an anode with higher capacity which uses silicon (Si), tin (Sn) or the like has been studied actively. However, due to expansion and shrinkage of an anode active material which occur when charge and discharge are repeated, the anode active material is cracked into small pieces, thereby the anode has extremely poor cycle characteristics such as a decline in current collecting performance, and promotion of the decomposition of an electrolyte solution due to an increased surface area. Therefore, an anode which includes an anode active material layer formed on an anode current collector through a vapor deposition method, a liquid-phase deposition method or a sintering method has been studied (refer to, for example, Japanese Unexamined Patent Application Publication No. Hei 8-50922, Japanese Patent No. 2948205 and Japanese Unexamined Patent Application Publication No. Hei 11-135115). According to the anode, compared to a conventional coating type anode formed through coating with slurry including an anode active material in a particle shape, a binder and so on, the anode can be prevented from being cracked into small pieces, and an anode current collector and an anode active material layer can be formed as a unit, so electronic conductivity in the anode becomes very good, and in terms of capacity and cycle life, the anode is expected to have higher performance. Moreover, a conducive material, a binder, a gap and so on conventionally included in the anode can be reduced or eliminated, so the anode can be formed into a thin film in essence.
However, even in the anode, due to intense expansion and shrinkage of the anode active material upon charge and discharge, the anode active material layer is cracked into small pieces, and a decline in current collecting performance, and promotion of the decomposition of the electrolyte due to an increased area surface occur, thereby resulting in a loss in the capacity. Therefore, the cycle characteristics in the anode are not sufficient enough. Moreover, due to intense expansion and shrinkage of the anode active material layer, linear and large cracks are produced in the anode active material layer, thereby when the strength of the anode current collector is not sufficient, a fracture is produced in part of the anode current collector, so sufficient cycle characteristics cannot be obtained accordingly. In order to secure the strength of the anode current collector, the anode current collector is required to have a larger thickness, but in terms of higher capacity, the anode current collector preferably has a smaller thickness.